Optical transmission system and noise suppression method

A noise suppression method is used in an optical transmission system which includes node equipments transmitting a WDM optical signal. First node equipment drops a first optical signal from the WDM optical signal. The method includes: transmitting a failure signal that specifies the first optical signal from second node equipment to the first node equipment via a first route when the second node equipment arranged on the first route for transmitting the first optical signal to the first node equipment has detected a failure of the first optical signal; transmitting a shutoff signal that specifies the first optical signal from the first node equipment toward the second node equipment via the first route; and shutting off an optical output of a wavelength corresponding to the first optical signal in third node equipment that has relayed the failure signal and has received the shutoff signal.